It is well known in the field of video displays to generate pictures on a screen by combining multiple beams of light. For example, a typical rear projection color television set includes three cathode ray tubes (CRTs), each CRT processing one of the primary colors--red, blue or green. By combining the three monochromatic beams the set can produce full color television pictures. However, in order for the set to produce accurate pictures, proper alignment of the beams must be maintained. That is, the CRTs must be calibrated so that their beams are focused at the same point on the screen. Accordingly, the calibration of the CRTs is often referred to as a convergence procedure, and beam alignment is often referred to as convergence. For a more detailed discussion of convergence, references are made to FIGS. 1 and 2.
FIG. 1 is a plan view of a model rear projection television set. The components of the set are housed within a cabinet 10, and they include: a CRT 12, a lens 14, a mirror 16, and a screen 18. The model set includes three CRTs and multiple lenses for each CRT, although for clarity, only a single CRT and a single lens are shown in the figure. The light from the CRT passes through the lens and illuminates the mirror which, in turn, reflects the light onto the screen for observation by the viewer.
FIG. 2 illustrates the relationship between the three CRTs of the model set. As can be seen from the figure, CRTs 12, 20 and 22 are matched respectively with lenses 14, 24 and 26, and the CRTs are aligned so that their beams converge. To maintain the alignment of the beams one or more photosensors are typically provided at the periphery of the screen. An example is shown in FIG. 3.
FIG. 3 includes an arrangement of four photosensors, 28, 30, 32 and 34. The sensors are located inside the cabinet and are not visible to the viewer. Also, the sensors are located behind a screen frame 36, which is not part of the display screen, and therefore the sensors do not interfere with images displayed on the screen. Nevertheless, the sensors are located within the area that can be scanned by the CRTs.
FIG. 4A shows the relationship between sensors 28-34, screen 18, and a CRT scannable area 38 as seen from the viewer's perspective. For clarity the screen frame is not shown. When performing the convergence procedure, test patterns are produced within the scannable area and detected by the sensors. More specifically, each CRT produces two test patterns, a wide pattern and a narrow pattern. Thus, to complete the convergence procedure the following patterns are produced: red-wide, red-narrow, blue-wide, blue-narrow, green-wide, and green-narrow. These patterns and their function are discussed in more detail in connection with FIGS. 4B-4E.
FIGS. 4B-4E show illustrative test patterns as generated by any one of the primary color CRTs. In the interest of brevity, FIGS. 4B-4E are discussed in the context of the red CRT only. However, it should be noted that the discussion is equally applicable to the other primary color CRTs.
FIGS. 4B and 4C show test patterns that are generated when the red CRT is properly aligned with the center of the screen. FIG. 4B shows a red-wide pattern and its relative position to the scannable area, screen, and sensors. As can be seen from the figure, the red-wide pattern is made up of four illuminated areas 40 that define a rectangle (indicated by the dotted line). Each illuminated area 40 overlaps the entirety of one sensor. The center point of the scannable area is denoted by "o" and the center of the rectangle defined by the red-wide pattern is denoted by "x". Since the red CRT is properly aligned, the o and x coincide.
FIG. 4C shows a red-narrow pattern with illuminated areas 42. As in the case of the wide pattern, since the CRT is properly aligned, the x and o coincide. However, in the case of the narrow pattern, only one half of each of the sensors is overlapped by the illuminated areas 42 of the pattern. The relative sensor overlap in the wide pattern and narrow pattern cases is key to maintaining alignment of the CRT, and will be discussed in more detail below. First, FIGS. 4D and 4E are referred to in order to show the effect of misalignment on the test patterns.
FIG. 4D shows a red-wide pattern 44 that is generated when the red CRT is misaligned by an amount .delta. in the horizontal direction (left of center from the viewer's perspective). Since the pattern is sufficiently wide, it still overlaps the entirety of each of the sensors. FIG. 4E shows red-narrow pattern 46 that is generated when the red CRT is misaligned by an amount .delta. in the horizontal direction (left of center from the viewer's perspective). In FIG. 4E, since the pattern is narrow, the sensor overlap is changed relative to the overlap shown in FIG. 4C. As will be described below, this change in overlap is used to determine the amount of misalignment, which is, in turn, used as an error signal for the purpose of correcting the misalignment.
The amount of beam misalignment at a position defined by a given sensor is determined by observing that sensor's readings when exposed to the wide and narrow patterns. The observed readings are used to form a ratio which is then compared to a desired ratio, the desired ratio being the ratio obtained for the sensor under no misalignment conditions. The difference between the measured ratio and the desired ratio indicates the amount of beam misalignment. Described below is an illustrative misalignment determination as performed by sensor 28.
FIGS. 5A-5E show the relationship between sensor 28 and various test patterns. FIG. 5A depicts the sensor in a no pattern condition. FIGS. 5B-5E show the sensor as illuminated by the patterns of FIGS. 4B-4E, respectively. To measure the misalignment, the light incident on sensor 28 is measured for each of the wide and narrow cases and a ratio of the two is computed. The value of the ratio in the no misalignment case is the desired ratio, and it is obtained in the following manner: the sensor reading under no pattern conditions (noise) is subtracted from the sensor reading under wide-pattern/no-misalignment conditions (FIG. 5B) to generate a first difference; the sensor reading under no pattern conditions is subtracted from the sensor reading under narrowpattern/no-misalignment conditions (FIG. 5C) to generate a second difference; and the second difference is divided by the first difference. To obtain the value of the ratio for the depicted misalignment: the sensor reading under no pattern conditions (noise) is subtracted from the sensor reading under wide-pattern/.delta.-misalignment conditions (FIG. 5D) to generate a first difference; the sensor reading under no pattern conditions is subtracted from the sensor reading under narrow-pattern/.delta.-misalignment conditions (FIG. 5E) to generate a second difference; and the second difference is divided by the first difference. The difference between the two ratios thus obtained indicates the amount of misalignment. The red CRT is then adjusted until the ratios match. A similar procedure is executed for the other primary beams and in this way convergence is achieved.